Traditionally, a liquid crystal display (LCD) represents a flat cell comprising two plane-parallel base plates and a system of electrodes. The electrodes are made of an optically transparent, conducting material, for example, a solid solution of indium and tin oxides (ITO), In2O3 and SnO2, and formed on the inner surfaces of the plates. The surface of the plate with electrodes is usually coated with a layer of polyimide or some other polymer and subject to a special treatment ensuring required homogeneous alignment of the molecules of a liquid crystal at the surface and in the bulk of a liquid crystal layer formed in the cell. After assembly, the cell is filled with a liquid crystal (LC), which forms a 5 to 20 μm thick layer functioning as an active medium, the optical properties of which change under the action of an applied electric field. The change in the optical properties is registered by a system of crossed polarizers usually glued onto the external surface of the cell. See Yeh, Pochi, and Gu, Claire, Optics of liquid crystal displays, N.Y. 1999.
Commercial LCD panels are fabricated using plates comprising a plane-parallel glass substrate and a conducting layer. The glass plate must be highly planar and free of bubbles and other optical defects. Depending on the future operation conditions, the blank plates are provided with different conducting layers. For displays operating in the transmission mode, the conducting layers are made transparent. For displays operating in the reflection mode, the front plate is made of a transparent conducting layer, and the back plate is made of a reflecting conducting layer. See Wu, Shin-Tson, and Yang, Deng-Ke, Reflective Liquid Crystal Displays, N.Y., 2001. Transparent conducting layers typically possess a surface resistivity in the range from 10 to 102 Ohm and a transmission coefficient of 0.7 to 0.9 in the visible spectral range. The conducting layers are fabricated by various deposition techniques.
Generally, an LCD panel bears an array of LCDs, with a necessary electrode configuration for each LCD fabricated by methods of mask etching. The electrodes terminate at the glass plate edges by contact areas suited for soldering external connectors. Individual LCDs on the panel are separated by etched paths, at which the desired LCD panel configuration will be glued. The necessary gap between plates is provided by spacers disposed at the perimeter of the assembly. An LCD panel (or array) is filled with an LC solution in vacuum. During this process, the plates are heated in order to decrease the LC viscosity and provide better filling of the gap. Then, individual LCDs formed in the array are separated by scribing and breaking, and each LCD cell is hermetically sealed. Finally, polarizers are glued onto the external LCD surfaces. The polarizers either bear a protective layer or are protected by additional glass plates.
In order to prevent diffusion of ions from glass to LC in the course of display operation, a special protective layer is formed between the glass plate and the conducting film. Generally, this protective layer is composed of silicon dioxide or a heavy metal oxide, although polymer films can also be used. The protective layer must be transparent in the working spectral range, while the layer thickness and density must provide for reliable isolation of LC from glass.
There are various methods for depositing protective layers, including physical techniques based on evaporating or sputtering materials, and chemical methods employing chemical reactions. See Proceedings of the 3rd International Conference on Coatings on Glass, Oct. 29-Nov. 2, 2000, Maastricht, the Netherlands. At present, most widely used technologies employ the method of vacuum deposition. This technique is based on the thermal evaporation of a given material under high vacuum conditions. The vapor condenses to form a thin layer on a substrate surface. The process is quite rapid, taking from a few seconds to several minutes.
Another physical deposition technique is based on cathode sputtering. This process employs atoms sputtered from a cathode bombarded with high-energy ions of a rarefied gas. The sputtered atoms are deposited onto a substrate surface. In the case of reactive sputtering, a chemically active gas (for example, oxygen) is introduced to the working chamber, which favors the formation of deposits with a required chemical composition.
Chemical deposition methods include, for example, the formation of films from solutions of hydrolyzed compounds. According to this, a silicon dioxide film can be obtained by sedimentation from a silicon ethylate solution onto a substrate rotating in a centrifuge.
U.S. Pat. No. 5,358,739 describes a method for the formation of a silicon dioxide film by applying a layer of silazane polymer onto a substrate, followed by heating in an oxidizing medium. There are many other methods for the formation of protective coatings.
Traditional polarizers for LCDs represent polymer films made of PVA or its derivatives oriented by uniaxial stretching, bulk stained with iodine compounds. See M. M. Zwick, J. Appl. Polym. Sci., No. 9, 2393-2424 (1995). The iodine-stained PVA-based polarizers possess high polarization characteristics in the longwave region of the visible spectral range and are highly stable in light. Disadvantages of the iodine-containing polarizers are low moisture resistance and thermal stability, which require using various protective layers. Such polarizers may contain up to ten layers.
An alternative to the iodine-containing polarizers is offered by the films containing dichroic dyes. See U.S. Pat. No. 5,340,504 and JP 59,145,255. Such polarizing films are obtained by uniaxially stretching a polymer (for example, PVA) film bulk-stained with a dichroic dye. However, use of the polarizers stained with dichroic dyes also requires applying protective layers.
The optical axis of polarizers obtained by uniaxial stretching is directed along one of the film sides. However, some designs of LCDs of the TN and STN types require that the polarization axis make a nonzero angle with the LCD sides. Cutting such polarizers leads to a considerable (up to 20%) amount of wastes.
Polarizers are glued onto an LCD cell after assembly, which introduces additional, technological operations and increases the cost of final products.
WO 94/28073 describes a polarizer based on a liquid-crystalline solution of organic dyes. According to this technology, the polarizer is obtained by depositing a thin layer of the liquid-crystalline dye solution onto a glass or polymer substrate by one of the known methods. A special feature of this technology is that the orientation of the dye molecules is effected in the course of deposition, so that a thin thermally stable polarizing film is obtained immediately upon drying. Using such polarizers allows new types of LCDs to be developed, in which the polarizing layers can be formed immediately on the walls (either external or internal) of the LC cell. The internal layers are preferred, since this design increases the strength and reliability of LCDs, simplifies the LCD design, and reduces the number of technological operations.
By properly selecting the application and alignment conditions, it is possible to obtain a dichroic polarizer containing anisotropic film possessing, at least partly, a crystalline structure. See EP 01128192 A1. Such dichroic polarizers are characterized by a higher degree of anisotropy and better thermal stability.
In LCDs with internal polarizers, a dichroic polarizing layer is usually formed above the electrode system according to EP 01004921 A1. For this purpose, the electrodes are covered with a special layer ensuring leveling (planarization) and improving adhesion of the dichroic polarizer material. However, this increases the number of layers, the total LCD thickness, and the number of technological operations. Moreover, application of a dichroic polarizer after the formation of electrode system makes the production scheme less flexible, thus hindering changes in the assortment of products.
Another problem in the production of LCDs is their protection from UV radiation. As is known, liquid crystals employed in modern LCDs degrade after 20 to 25 hours upon exposure to the UV radiation. This necessitates the use of additional protective layers or special materials in the LCD design. See U.S. Pat. No. 5,539,552. For example, U.S. Pat. No. 5,281,562 describes special glasses possessing a sharp cutoff at about 400 nm used as materials for the base plates.